This invention relates to a compact, wide-angle collimating optical device and, more particularly, to such a device utilizing a linear polarizing beamsplitter.
A goal of modern military and police forces is to operate as effectively at night as in daylight. To accomplish this goal, night vision for troops is an important asset. Night-vision devices have been developed either to intensity available light in the visible band or to detect images using infrared energy. These devices are moderately effective when measured in terms of optical efficiency. Progress has also been made in reducing their size, weight, power consumption, and cost, and increasing their robustness and comfort for the user.
As the night-vision devices have been improved, their requirements have been expanded simply from the ability to visualize a scene at night to xe2x80x9csituational awarenessxe2x80x9d. One aspect of situational awareness is a wide field of view for the user of the device. Existing narrow-field-of-view night vision devices impart the sensation of looking down a narrow tunnel, so that the user does not have the advantages of the peripheral vision and other daylight capabilities that allow the user to be aware of events over a wide forward-looking field.
The available night-vision devices with moderate optical efficiency are limited to narrow fields of view of only a few degrees. The available night-vision devices with a wider field of view have poor optical efficiency. There is, accordingly, a need for an improved night-vision device with both a moderate-or-better optical efficiency, and a wide-angle field of view approaching 90 degrees. The present invention fulfills this need, and further provides related advantages.
The present invention provides a wide-angle optical collimating device which has moderate optical efficiency in one highly robust embodiment, and excellent optical efficiency in a less-robust embodiment that is acceptable for many applications. The maximum field of view of the device is on the order of about 90 degrees, giving the user much more awareness of the surroundings than possible with available narrow-view night-vision devices. The optical collimating device of the invention is light in weight and compact, making it useful for helmet-mounted or head-mounted display systems.
In accordance with the invention, a wide-angle collimating optical device comprises a first absorptive linear polarizer having a front face and a back face, a first quarter-wave plate having a front face in facing relation to the back face of the first absorptive linear polarizer, and a back face, and an optical doublet having a front face in facing relation to the back face of the first quarter-wave plate, and a back face. The optical doublet comprises a plano-concave singlet whose planar face forms the front face of the optical doublet and whose concave surface has a curvature thereto, a plano-convex singlet whose planar face forms the back face of the optical doublet and whose convex surface has the same curvature as the curvature of the concave surface, and a first reflective beamsplitter joining the concave surface of the plano-concave singlet to the convex surface of the plano-convex singlet. There is a second reflective beamsplitter having a front face and a back face. Exactly one of the first reflective beamsplitter and the second reflective beamsplitter is a linear polarizing beamsplitter, preferably a wire grid polarizer, and neither the first reflective beamsplitter nor the second reflective beamsplitter is a cholesteric liquid crystal. There is additionally a second quarter-wave plate having a front face and a back face. The front face of the second quarter-wave plate is in facing relation to the back face of the optical doublet and the back face of the second quarter-wave plate is in facing relation to the front face of the second reflective beamsplitter. The plano-concave singlet and the plano-convex singlet are each preferably made of glass. There may additionally be an infrared detector adjacent to the front face of the first absorptive linear polarizer.
In one form, the first reflective beamsplitter is the linear polarizing beamsplitter. In another form, the second reflective beamsplitter is the linear polarizing beamsplitter. In either case, there may be a second absorptive linear polarizer having a front face in facing relation to the back face of the second reflective beamsplitter. In a compact, rugged version, the back face of the first absorptive linear polarizer is bonded to the front face of the first quarter-wave plate, the back face of the first quarter-wave plate is bonded to the front face of the optical doublet, the back face of the optical doublet is bonded to the front face of the second quarter wave plate, and the back face of the second quarter wave plate is bonded to the front face of the second reflective beamsplitter.
Further in accordance with the invention, a wide-angle collimating optical device comprises an optical doublet having a front face and a back face. The optical doublet comprises a plano-concave singlet whose planar face forms the front face of the optical doublet and whose concave surface has a curvature thereto, a plano-convex singlet whose planar face forms the back face of the optical doublet and whose convex surface has the same curvature as the curvature of the concave surface, and a reflective beamsplitter joining the concave surface of the plano-concave singlet to the convex surface of the plano-convex singlet. The reflective beamsplitter comprises a linear polarizing beamsplitter. There are additionally a quarter-wave plate having a front face in facing relation to the back face of the optical doublet and a back face, and a cholesteric liquid crystal circular-polarizing beamsplitter having a front face in facing relation to the back face of the quarter-wave plate. The plano-concave singlet and the plano-convex singlet are each preferably made of glass. There may additionally be an infrared detector adjacent to the front face of the optical doublet.
Preferably for compactness and robustness, the back face of the optical doublet is bonded to the front face of the quarter-wave plate, and the back face of the quarter-wave plate is bonded to the front face of the cholesteric liquid crystal circular-polarizing beamsplitter.
There may additionally be an absorptive linear polarizer having a front face, and a back face in facing relation to the front face of the optical doublet. In this case, the back face of the absorptive linear polarizer is bonded to the front face of the optical doublet, the back face of the optical doublet is bonded to the front face of the quarter-wave plate, and the back face of the quarter-wave plate is bonded to the front face of the cholesteric liquid crystal circular-polarizing beamsplitter.
In the first approach, one of the reflective beamsplitters is preferably a linear polarizing beamsplitter and the other is a beamsplitter coating. No cholesteric liquid crystal is used, so that the device does not have the field-of-view, robustness, cost, color, and temperature limitations associated with the cholesteric liquid crystal. This device has an optical efficiency on the order of about 10 percent, and a wide field of view on the order of about 90 degrees.
In the second approach, a cholesteric liquid crystal is used in conjunction with a linear polarizing beamsplitter. The device is limited by the performance and cost of the cholesteric liquid crystal, and has a more limited field of view of about 60 degrees. However, it has an extremely high optical efficiency on the order of about 30 percent.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.